Why six hours of sleep is not enough

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For those of us who worry about our sleep, every morning is a scramble of mental maths as we turn to our bedside clock and calculate whether or not we've had 'enough'.
The current National Sleep Foundation guidelines recommend that most adults sleep for between seven and nine hours per night. This followed a 2022 paper from the University of Cambridge, itself an amalgam of hundreds of studies that followed people's long-term experience of heart disease, diabetes and mental-health difficulties.
'Having a consistent seven hours' sleep each night, without too much fluctuation in duration, was also important to cognitive performance and good mental health and wellbeing,' say the authors of the paper. Those who slept between seven and nine hours reported a lower incidence of these chronic conditions.
But what if, for any reason, you consistently don't sleep that much? If you have trouble sleeping, or if you just want to jump onto the latest 5am trend, get up with the lark and to find a couple more precious hours in your day? Does it have real implications – and does it even matter at all?
What is the problem with having six hours sleep?
The long-term impacts of sleep deprivation
So are people who only sleep six hours a night doomed?
Strategies to improve sleep quality
If you consistently have less than six hours' sleep, the chances are that you are risking health problems.
'On the surface, you will be moody, frustrated and irritated,' says Maryanne Taylor, a sleep consultant from sleep consultancy The Sleep Works. 'On a deeper level, you'll be raising your risks of stress and anxiety – these have a bidirectional relationship to your sleep.' Three key problems are:
Sophie Bostock is a sleep scientist and behavioural psychologist. 'We know that poor sleep and mental health are inextricably linked,' she says. 'Poor sleepers are twice as likely to develop anxiety and depression than good sleepers.
'While this is no surprise to those who lie awake at night worrying, there is some good news, too – improving one's sleep will benefit one's mental health.'
'Our brains evolved to see sleep loss as a warning sign,' says Bostock. 'Our ancestors were more likely to be kept awake by predators than by scrolling through their phones, so we respond to short sleep by going into high alert.'
Hence, we get more sensitive to potential foes.
'We are more likely to interpret neutral faces as threatening and to withdraw from social situations, diverting resources from the more rational part of our brain and decision-making capacity,' she says.
'Without enough sleep, in the longer term, we struggle to focus, to learn and remember, to empathise and make logical, sensible decisions,' says Bostock.
Insomnia is linked to a wide variety of conditions, from obesity to Type 2 diabetes and even Alzheimer's. The reasons were discussed in the May 2019 edition of the journal Experimental Psychology, with theories ranging from blood vessels littered with fatty deposits, to 'cellular garbage' in the brain.
According to the research, people who sleep fewer than seven hours a night have a significantly raised level of molecules called microRNAs, which suppress the protein content of cells and have previously been linked to inflammation and poor blood vessel health.
Not getting enough sleep makes you more likely to gain weight, according to the analysis of 36 studies, discussed in the journal Obesity. Various studies have shown that poor sleep disrupts the productions of the hormones ghrelin and leptin – the hormones that control hunger. Thus, insomniacs crave fatty, starchy and sugary foods, potentially eating hundreds of extra calories a day in refined carbohydrates
The daytime exhaustion means you can't be bothered to exercise and so your weight gain spirals, as does a cascade into other conditions such as diabetes and heart disease.
People with a chronic lack of sleep significant risk an increase in Type 2 diabetes, according to a report in the journal Diabetes Care.
Patients who suffered poor sleep (defined as fewer than five hours a night) for a year or longer had three times the risk of those who slept six hours or more.
As with obesity (also linked to Type 2 diabetes), the underlying cause is thought to involve disruption of the body's normal hormonal regulation, but in this case it results from insufficient sleep.
It's well known that sleep is necessary for a healthy immune system, the part of our make-up which tackles antigens, or foreign invaders, as well as 'T cells': white blood cells which destroy virus carrying cells.
One study showed that subjects who slept fewer than seven hours a night were three times as likely to catch colds than those who got the full eight, or more.
Much modern research indicates that insomnia raises the risk of Alzheimer's. The Harvard Medical School report revealed that people with sleep impairments were nearly 1.7 times as likely to develop cognitive impairments than those without.
Researchers from the University of Tennessee noted that chronic insomnia was tied to a 1.5 times higher risk of kidney decline, and an even steeper increase in risk of kidney failure: 2.4 times.
A comprehensive report linking insomnia with high cholesterol and blood pressure was published in 2019 by a top American university.
People who don't get enough sleep also have raised levels of stress hormones and substances that indicate inflammation, a key cause of cardiovascular disease. Fewer than four hours' a night sleep could double a woman's risk of dying of heart disease.
But before you take to your bed in despair, Taylor draws a distinction between short periods of six hours' sleep or fewer, and longer-term sleep deprivation. 'If a small burst of stress (a job interview, for example) impacts the amount of hours you get, it's unlikely to mean you have an ongoing sleep problem. We all have these blips.'
Bostock also says there is no reason to panic. 'All these chronic conditions are multifactorial, so we need loads of subjects to work out the rule of the impact of confounding factors. In the short term studies, typically healthy people recover after several days of proper sleep.'
The first step is to make sure you have the bases covered: that your bedroom is dark, quiet and at optimum temperature – between 16C to 18C. Your bedding should be comfortable and, ideally, made of natural fibres. Cut out coffee after midday, and have a proper wind-down routine.
If you are suffering with chronic sleep problems, attending to sleep hygiene may not be enough, however. 'We can become a slave to hygiene and in some ways it can be counterproductive,' says Taylor.
'There is so much paraphernalia out there: Spray this! Drink this! The fact is that people who sleep well don't actually do anything. Sleep is an innate process, and we all can do it: it's just about creating the conditions in our brain.'
Instead, Taylor suggests adopting a more flexible mindset. 'Bedtime procrastination can cause sleep anxiety,' she says. 'Firstly, it really is OK to stop looking at your work emails after 7pm.'
Waking up in the night is part of the natural process, says Bostock. 'Your sleep goes in 90-minute cycles, and people tend to wake up more as they get older.'
But problems arise if you wake up and have difficulty falling back to sleep again. 'Many people who wake up at night tell themselves: I have to stay in bed,' says Taylor. 'But this can be the crux of maintaining sleep problems.'
Taylor suggests making what she calls a 'nest' outside your bed. 'A comfortable chair or beanbag, lit with dim light, with a book or your headphones, and not your phone,' she says. 'When you feel sleepy, go back to bed. If you have to get up a few times a night to do this, that's fine.'
Anyone who's had a run of broken nights will agree with Taylor's philosophy that sleep doesn't do well under pressure.
'Have a look at your sleep narrative,' she says. 'If you are telling yourself: 'I'm a terrible sleeper', 'I won't be able to manage tomorrow,' this can be very powerful and only serve to make you more stressed and less able to drop off. You need to step out of your own way.'
Instead, tell yourself: 'It's OK, I will survive', and 'I'm a good sleeper, I'm just having a bad night.'
Sleeping well is not just about what happens in the hours of darkness: you set yourself up for the next night the minute you open your eyes that day.
'What do you do the minute you wake up?' asks Taylor. 'If you feel rubbish, do you keep pressing snooze until you jump up in a panic? Do you pick up your phone, and are catapulted brutally into the next day? Of course your sleep will be impacted.'
Instead, she recommends easing into the day with a cup of tea, in the light. 'As soon as you wake up, aim to get lots of light into your eyes,' she says. 'Go outside if you can, if not, or at least sit near a window. This banishes the sleep hormone, melatonin, and activates the cortisol awakening response in your brain telling you that the day has begun.'
Miranda Levy is the author of The Insomnia Diaries: How I Learned to Sleep Again
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